钽铌酸锂晶体的布里渊散射研究

研究了钽铌酸锂(LiTa_xNb_(1-x)O_3,x=0.0013,0.0052,0.015,0.06)晶体在室温的布里渊散射.给出了它们的弹性刚度常数和压电应力常数.测量了一种组份晶体的布里渊散射频移在50K～580K范围内的温度特性.发现某些声子散射谱中除三个正常峰外还存在一个异常峰.     

Electron-phonon scattering and conductivity of a quantum cylinder in a magnetic field

The contribution of electron-phonon scattering to conductivity of a quantum cylinder in a longitudinal magnetic field has been studied. It has been shown that the conductivity of the nanotube undergoes Aharonov-Bohm oscillations with variations in the magnetic flux through the nanotube cross section. The formulas describing the temperature dependence of the resistance of the nanostructure both in the case of an isotropic phonon spectrum and with allowance for the effects of phonon confinement have been obtained in the analytical form.     

On the two-magnon scattering of light and neutrons

The effect of magnon-magnon interaction on the two-magnon scattering of light and neutrons has been calculated at finite temperatures. Neglecting life-time effects the temperature dependence of the magnetization and that of the spin wave energy as well as the Bose functions for the magnon occupation number have been taken into account. In the low temperature region, where this spin wave approximation is valid, we have gotten a shift of the two-magnon peak toward smaller energies with increasing temperature, which scales with the temperature dependence of the magnon energy and a broadening of the peak as well. These features are in agreement with the result of optical experiments.     

Self-diffusion in liquid lithium and lead from coherent quasi-elastic neutron scattering data

The temperature dependence of the self-diffusion coefficient of liquid lithium in the temperature range of 500–830 K has been investigated using coherent quasi-elastic neutron scattering. Good agreement with the results obtained from incoherent quasi-elastic neutron scattering experiments traditionally used for this purpose has been achieved, as well as with the corresponding literature data. In addition, one temperature point (623 K) has been obtained for the self-diffusion coefficient of liquid lead, which is a pure coherent scatterer.     

Effect of dipolar forces on the response functions of ordered magnetic crystals and their light scattering spectra

Response functions for ferromagnets and antiferromagnets are calculated using the linearized Bloch's equations for the spin system including dipole-dipole interaction. This calculation has proved to be equivalent to the simplest random-phase decoupling approximation when dipolar forces are not considered. Light scattering cross-sections have been calculated on the basis of the response functions obtained. The effect of dipole-dipole forces has been shown to be important when the shift in the magnon spectrum introduced by them is of the same order of magnitude as the magnon energy in the absence of such forces. Y.I.G. and MnF₂ have been considered as examples of ferromagnetic and antigerromagnetic crystals respectively, for which the variation of the scattering cross-section with temperature and external magnetic field is given     

Resonance Raman scattering in InAs near the E1 gap

Measurements of resonance Raman scattering in InAs at 77°K near the E₁ gap have been extended to 2.73 eV. The peak in the resonance curve appears at about 2.66 eV, 70 meV above the optical gap, and gives a larger temperature shift of the resonance than previously reported. Resonance lineshapes are obtained for allowed TO and LO and for forbidden LO phonon scattering. The forbidden scattering intensities are consistent with selection rules predicted for linear q-dependent and/or surface electric field induced scattering mechanisms.     

Impurity scattering on the Fermi surface of aluminum

A calculation is described for the anisotropic relaxation time due to impurity scattering on the Fermi surface of aluminum. The Bloch states and the Fermi surface are obtained from Ashcroft's 4-OPW model, while the scattering potentials are obtained from locally re-screened form factors. Numerical results reveal that strong anisotropicsin the electronic lifetime arise for s-like impurities such as Si and Ge. A phase shift model is introduced to explain these anisotropies. The predicted anisotropies appear to be in reasonable agreement with experimental values for the Dingle temperature. The relaxation time for d.c. conductivity is obtained from iterative solution of the Boltzmann equation.     

Q-dependent light scattering by electrons in LaB6

The inelastic light scattering by intraband electronic excitations in metallic lanthanum hexaboride has been studied in the temperature range of 10–300 K. General agreement has been obtained between the measured spectra and the spectra calculated within the band theory taking into account the renormalization of electron energies owing to electron-phonon scattering. The electron-phonon coupling constant λ and electron relaxation frequency Γ have been estimated. The dependence of the electron self-energies on the direction and magnitude of the wave vector has been revealed, implying the anisotropic electron-phonon interaction or the contribution from other electron scattering mechanisms.     

Temperature shift of the maximum of critical X-ray scattering in a BaTiO3 monocrystal

The temperature shift of the maximum of critical scattering in BaTiO₃ monocrystal was found to depend on the direction of the scattering vector with respect to the crystalline axes.     

On electron-electron scattering in molybdenum

Results of the doppleron damping measurements of the temperature dependence of the scattering rate $\text{v}$(T) for carriers on the non-central sections of the different sheets of the Fermi surface (FS) of molybdenum are presented. The αT² law has been found over the temperature range 1.5–5 K, with α practically the same for the electron jack and hole octahedron. The values of α are in a good agreement with those obtained from the radio-frequency size effect data for the central sections of the FS. From the analysis of these data and results obtained by other experimental methods it was concluded that the quadratic terms in the temperature dependence of the scattering rate in molybdenum is due to the electron-electron scattering. A discussion of the observation of electron-electron scattering in the non-transition metals is given.     

Rayleigh scattering from a trapped bose condensate and the corresponding recoil atom velocity distribution

An approach has been developed that allows the Rayleigh scattering cross section to be calculated with allowance for the quantum character of motion of the center of mass of the trapped scattering particles. The shape of the line of light scattering from a Bose condensate in a parabolic trap has been studied. A shift of the scattering line center is equal to the recoil shift, while the line width depends on the chemical potential of the Bose gas and on the relaxation time of the velocity of the above-condensate recoil particles. A velocity distribution function in the beam of recoil atoms formed in the course of induced Rayleigh scattering is determined. It is shown that, under the typical experimental conditions, the characteristic width Δv/v of the recoil velocity distribution in this beam is on the order of 10^?3 at a velocity v on the order of several centimeters per second.     

Low-order light scattering in multiple scattering disperse media

Light scattering has been investigated in systems in which both single and higher order scattering occur. The Monte Carlo simulation technique for studying light scattering in randomly inhomogeneous, strongly scattering disperse media was employed. The reliability of the data obtained has been checked by comparing the results of the computer simulation with analytical calculations for the intensity of doubly scattered light. The first several scattering orders have been analyzed for different geometries of the optical experiment. It has been shown, in particular, that, depending on the detector aperture, the contribution of multiple scattering can vary by almost an order of magnitude.

     

Dielectric susceptibility of a deuterated KDP crystal from experiment on Raman scattering and in the cluster approximation

The temperature dependence of the static dielectric susceptibility obtained from the experiment on Raman scattering in a deuterated KDP (DKDP) crystal has been described using the cluster approximation in terms of the microscopic mean-field theory. The optimum values obtained for parameters of the cluster approximation are in good agreement with experimental data over a wide temperature range. These values are found to be comparable to the estimates previously obtained in describing experimental data on the DKDP crystal.     

Energy characteristics of photothermal vibrational instability at stimulated Raman and Mandelstam-Brillouin scattering in aerosol droplets

The threshold intensity and the combination frequencies for photothermal vibrational instability in high-Q aqueous aerosol droplets in the two-mode regime have been calculated. The selection rules for coupling electromagnetic and temperature modes in a droplet are obtained. A comparative analysis of the threshold excitation intensities of photothermal vibrational instability, stimulated Mandelstam-Brillouin scattering, and stimulated Raman scattering in droplets is performed. It is shown that photothermal vibrational instability in the two-mode regime can be developed at a pump intensity of about 10⁴ W/cm² for droplets with radii of 2–20 μm for a pump wavelength of 0.532 μm. A method of remote measurement of the microphysical droplet parameters from the additional periodic temperature shift of droplet eigenfrequencies in the spectrum of stimulated Raman scattering and lasing is proposed.     

Stimulated Brillouin scattering in highly birefringent microstructure fiber: experimental analysis

We report on an experimental analysis of stimulated Brillouin scattering (SBS) in a 20-m-long highly birefringent microstructure fiber for sensing applications. In particular, an experimental setup based on Brillouin optical frequency-domain analysis, operating at a wavelength of 1550 nm, has been employed in order to analyze the distribution of Brillouin frequency shift along the fiber, as well as to study the dependence of Brillouin frequency shift on optical polarization, temperature, and strain. Our results indicate that, for any fixed polarization, the fiber has a dual-peaked Brillouin spectrum. A study about the origin of these two peaks is presented.     

Single-photon emission computed tomography in a proportional scattering medium

The reconstruction of the spatial distribution of radiation sources in a proportional scattering medium has been considered. An exact solution of the inverse tomographic problem has been obtained by analytically solving the radiation transport equation for arbitrary distributions of radiation sources and extinction coefficient and exact boundary conditions. The type and scale of distortions associated with radiation scattering in tomographic reconstructions have been demonstrated.     

Stimulated thermal scattering induced by two-photon absorption and experimental observation of stimulated Brillouin scattering in the UV region

An analysis of the results of previous studies of stimulated scattering of UV pulses in liquids has shown that they disagree with the theory of stimulated scattering. To resolve the inconsistency, stimulated scattering of XeCl excimer laser radiation (λ = 308 nm) with pulse duration τ ≈ 8 ns in liquid hexane is investigated experimentally. A theoretical analysis of the results obtained revealed a new nonlinear optical phenomenon: stimulated thermal scattering induced by the heating due to two-photon absorption, called two-photon stimulated thermal scattering (two-photon STS-2). The stimulated backscatter spectrum contains a previously unknown line corresponding to two-photon STS-2 and a newly discovered SBS line in the UV region. The line is observed in experiment on liquid hexane and is characterized by the frequency shift Ω_B = 0.33 cm^?1 relative to the pump wavelength λ = 308 nm, in complete agreement with the theory of stimulated Brillouin scattering (SBS). The spectral line called the SBS line in previous studies has a frequency shift much smaller than that predicted by the SBS theory and must be interpreted as an unshifted two-photon STS-2 line. When two-photon STS-2 is used to obtain a phase-conjugate wave, the phase-conjugation fidelity is lower than that achieved by using SBS because of thermal self-action and slow decay of the thermal grating.     

Correlation between the quasi-elastic light scattering and the ionic-conductivity in superionic conductors

The quasi-elastic light scattering of the superionic conductors, (ZrO₂)_1?x (YO_1.5)_x (x = 0, 0.18, 0.33) has been studied in the region of frequency shift below 2.5 cm^-1. The quasi-elastic scattering far more intense than the Brillouin scattering has been found for a sample with x = 0.18. From the x-dependence of the intensity, it is shown that the ionic diffusion is responsible for the quasi-elastic scattering. The temperature variations of the intensity and the spectral shapes are interpreted in accordance with the macroscopic conductivity data, in terms of a model in which the barrier height distribution is taken into account.     

Collective microdynamics of liquid lithium: An inelastic neutron scattering study

A portion of the dispersion curve for collective modes in liquid lithium has been constructed from experimental data on inelastic scattering of slow neutrons obtained on the DIN-2PI neutron spectrometer (IBR-2 reactor, Joint Institute for Nuclear Research, Dubna, Russia). Measurements have been performed at a temperature of 500 K (T _m (Li) = 453.7 K). The coherent scattering component has been separated from the experimental spectra and analyzed. Information on the characteristics of collective excitations in liquid lithium has been derived.